The presently described technology relates to a method for driving matrix displays which are made up of a plurality of lines with individual pixels, which lines are configured as rows and columns, wherein individual lines are driven selectively by rows being activated for a defined row addressing time and an operating current or a corresponding voltage i.e. an electrical driving signal being applied to the columns in correlation with the activated row corresponding to the desired brightness in the pixels.
In the following text, the horizontal lines are referred to as rows and the vertical lines which run orthogonally to them are referred to as columns. This is for reasons of clarity. The invention is however not limited to exactly this arrangement. It is in particular possible to exchange the function of the rows and columns or select a non-orthogonal relationship between the rows and columns.
The image data or the desired brightness Dij of individual pixels ij are described with the matrix D shown below.
  D  =            (                                                  D              11                                                          D              12                                            …                                              D                              1                ⁢                m                                                                                        D              21                                                          D              22                                            …                                              D                              2                ⁢                m                                                                          …                                …                                …                                …                                                              D                              n                ⁢                                                                  ⁢                1                                                                        D                              n                ⁢                                                                  ⁢                2                                                          …                                              D              nm                                          )        .  
The indices correspond to the positions of the pixels on the display, which is given by the matrix or matrix display D. Each row i of the matrix D and each column j on the matrix D correspond in each case to the geometric row and column on the display. A pixel diode or similar element is assigned to each drivable pixel ij of the matrix display D for generating a pixel of a display. The light intensity averaged over time (corresponding to the brightness Dij) in each pixel corresponds with the corresponding element in the matrix D. All entries of the matrix D together produce the image to be displayed.
The pixels ij on the matrix display D, of which each can be configured in particular as an OLED (Organic Light Emitting Diode), have been activated so far by row. To this end, the OLEDs on a selected row i are activated by a switch by being connected e.g. to ground. An operating current I is impressed in each of the columns j, which current causes the pixels ij in the intersection of this row i and the columns j to light up. The light intensity L is in the first approximation proportional to charge which is impressed during the active phase (row addressing time) and is recombined radiantly in the OLED pixel. In the event of a relatively high frame rate of the addressing of the display matrix or matrix display D, the human eye perceives the following mean value of the intensity L of the light:
            L      Licht        ∼                            ∫          0                      T            Frame                          ⁢                              I            OLED                    ·                                          ⁢                      ⅆ            t                                      T        Frame              ≈                  I        ·                              T            Frame                    n                            T        Frame              =      I    n  
TFrame is the total time which is necessary for building a complete image if all n rows of the matrix display D are activated once. The operating current IOLED or I or I0 is impressed in each pixel. When amplitude modulation is used for controlling brightness, the operating current is active over the period TFrame/n which corresponds to the row addressing time. With pulse width modulation, the duration of the operating current is shorter, that is d*TFrame/n. Here, d is the pulse width modulation duty ratio and lies between zero and one:
      L    Licht    ∼      d    ·                  I        0            n      
The current I0 is now constant independently of the light intensity of the pixel. The intensity L is adjusted by means of the duty ratio d. A brightness control of this type is simpler and more precise in comparison to amplitude modulation, as the time units in the electronics can be adjusted very precisely and consequently d also. Just a reference current I0 is sufficient for driving all the pixels ij. With amplitude modulation, in contrast, the amplitude must be adapted in each case in correspondence with the desired brightness Dij.
By driving all the columns j of in each case only one row i, each diode or each pixel ij can be active only for a maximum of an nth of the total time TFrame. In order to achieve a defined average brightness Dij, the corresponding operating current must be multiplied by the number n of rows, that is, in contrast to the case in which one pixel would be supplied with operating current over the total time TFrame. That is, the higher the number of rows, the higher the pulsed operating current I or I0 must be. The operating current is moreover always high when pulse width modulation is used for adjusting the brightness, even when the pixel ij to be driven is very dark. In this case just the application time of the operating current is very short.
The high operating current can however lead to a significant reduction in the OLED lifetime. In order to achieve the necessary high operating current, the voltage at the OLEDs must also be increased, as a result of which the power consumption increases and the efficiency is reduced. This increased power loss not only discharges the rechargeable or disposable battery more quickly, but also makes the display warmer, as a result of which the lifetime is likewise reduced.
In order nevertheless to realise a large, highly resolving display, an “active matrix” could be used as in LCDs (Liquid Crystal Display), by means of which the operating current is no longer delivered in a pulsed manner but is present as a constant current. Active matrix driving (TFT backplane) however requires significant additional costs for an OLED display.